> Has there been any consideration to create AI to work around pathological cases in tracing? None of which I'm aware. > If compiling is done on a secondary t

> Has there been any consideration to create AI to work around pathological cases in tracing? None of which I'm aware. > If compiling is done on a secondary thread, aren't you then reaping all the benefits of JIT optimization while still not paying the penalty on the actual JS execution thread? Compilation can be done off-thread, but to decide what to compile you have to do some on-thread data collection, and until you're done compiling you have to run in a lower tier. Also, you usually have to do some work to make sure that your speculative optimizations are correct: for example, an inline cache needs to verify that you've got a cache hit. > Why throw away optimized code? Why not add a type check on the parameters and then dispatch to the highly-optimized code version based on the parameter count and types? Optimized code is optimized under a certain set of assumptions, which can be propagated through a function. For example, if the value that we read from an object's property is always an int, then we can do integer arithmetic with it afterwards. If we suddenly load a string or a double, then a bunch of downstream code will also be invalid. It's cheaper and less error prone to start over, rather than trying to track all the places that depend on a particular piece of information. If you only specialize individual operations based on the type input, and don't propagate type information between operations, they you've basically got polymorphic inline caches. That's what we use in our baseline compiler.